System and method for making records



y 1954 F. L.BLAKE 3,142,562

SYSTEM AND METHOD FOR MAKING RECORDS Filed Dec. 3. 1959 2 Sheets-Sheet 1/0 FIG 4 mmvron Heder/b/r L. B/a/re y 28, 1964 F. L. BLAKE 3,142,562

SYSTEM AND METHOD FOR MAKING RECORDS Filed Dec. 3, 1959 2 Sheets-Sheet 2VOLTAGE SOURCE 25 VOLTAGE 22 SOURCE F69 MM 1 VOLTAGE SOURCE IN VEN TOR.Frederick L. B/a/re AID/5 United States Patent 3,142,562 SYSTEM ANDMETHOD FOR MAKING RECORDS Frederick L. Blake, Scottsdale, Ariz.,assignor to Motorola, Inc, Chicago, 111., a corporation of IllinoisFiled Dec. 3, 1959, Ser. No. 857,159 8 Claims. (*Cl. 961) This inventionrelates generally to electrical recording sytsems. In particular, itrelates to a combination electrical-photochemical-catalytic system inwhich local areas of a recording medium are acted on by light energy andcatalyst material which is moved to the recording medium by anelectrical field, to provide records in the form of printing, designsand images, for example.

The catalytic system and materials of the invention can be applied invarious arts and in various physical forms. One application, forexample, is telegraphic and radio message transmitting systems of thetype wherein messages are printed on a recording medium in the form of asheet by applying electrical energy to the sheet through a scanningstylus or other marking device. In one Well known type of recordingsystem for such applications, the record on the sheet is formed by anelectrolytic chemical reaction which produces a colored product, and theenergy which causes the electrolytic reaction is supplied by the currentwhich is applied to the sheet by the stylus.

Photography is another application for electrical recording systems. Inknown electrophotography systems, current which is area-modulated inaccordance with the light and dark variations of the subject passesthrough a recording medium, and this modulated current causes anelectrolytic chemical reaction which produces dark and light elementalareas on the medium in the form of an image. The recording medium isusually only one layer in a multilayer structure, which also includessuitable electrodes and a photoconductive layer that provides thecurrent modulation mentioned above.

The sensitivity of electrolytic recording systems is limited by the factthat the electrolytic reaction requires a specific amount of current inaccordance with Faradays law, and no practical way of avoiding thislimitation on sensitivity has been developed. With respect to messageprinting applications, this means that the printing speeds which can beobtained are limited, and for electrophotography applications it meansthat the film is slow and requires long exposures. Of course, thecurrent can be increased by raising the applied voltage in order tospeed up the electrolytic reaction, but unduly high voltages may causesparking and even breakdown of the recording sheet. More current can beused if the electrical resistance of the recording sheet is minimized,and this can be accomplished by wetting the sheet with a liquidelectrolyte. However, an unduly wet sheet is difficult to handle, andspecial storage facilities or special Wet processing is required to makethe sheet ready for use. Although dry electrosensitive recording sheetsare available for facsimile and printing applications at the presenttime, these are not sufiiciently sensitive for many applications, andthey usually cause undesirable sparking.

It is one object of the present invention to provide an electricalrecording system in which the action of catalyst material affords a gainin sensitivity as compared to wholly electrolytic recording systems.

Another object of the invention is to provide an electrical recordingmedium which is substantially drier than known electrolytic recordingmediums and which is more sensitive than such mediums.

Another object of the invention is the provision of sensitive materialsfor an electrical recording medium which are stable in normal ambientconditions, and which are rendered highly unstable only at the time therecord is formed so that the medium can be stored and handled in apractical manner.

A further object of the invention is to provide materials andcompositions for an electrocatalytic recording system which aresufficiently noncritical with respect to the amounts, purity and methodsinvolved in the formulation thereof as to permit economical manufactureof the recording medium on a commercially practical basis.

A feature of the invention is the provision of a recording medium with astable coloring ingredient which undergoes a rapid photochemical changeto a contrasting color when contacted by catalyst material and also exposed to radiation such as light. Since it is the combination of lightand catalyst which produces the color change rather than electricity,the sensitivity of the recording medium is not limited by Faradays law.

Another feature of the invention is the provision of a recording mediumas described in the preceding paragraph wherein a small amount of thecatalyst material causes a larger amount of the coloring ingredient tochange in color rapidly in the presence of radiation such as light,thereby providing a gain in sensitivity.

A further feature of the invention is the provision of an electricalrecording system as described above in which the catalyst material isbrought into efiective contact with the coloring ingredient of therecording medium by electrodepositing photosensitive ions of thecatalyst onto the recordin medium, and converting the deposited ions tothe active catalyst by exposure to light. Since it is the combination ofelectrical energy and light energy which places the catalyst ineiiective contact with the coloring ingredient, either the light or theelectricity can be modulated so as to control the coloring process suchthat images, designs, printing or other records are formed on therecording medium.

Another feature of the invention is to provision of a sensitive coloringingredient for a recording medium which is a water insoluble,crystalline pigment having a stable state of characteristic color, andwhich changes rapidly to a contrasting color when contacted by catalystmetal and exposed to light rich in ultraviolet. The coloring ingredientis comparatively insensitive to light and other radiation in the absenceof the catalyst metal, so the recording medium can be stored, shippedand used without special precautions for protecting it from suchradiatron.

A still further feature of the invention is the provision of a metalcatalyst for the recording system described above which has acatalytically inactive ionic form that is convertible to thecatalytically active free metal by exposure to light, thereby permittingthe formation of images on the recording medium by optical modulation.

Certain embodiments of the invention are illustrated in the accompanyingdrawings in which:

FIG. 1 is a view of a recording sheet in the form of a homogeneous filmin accordance with one embodiment of the invention;

FIG. 2 shows a recording sheet comprising a coating on a conductivesupport which forms another embodiment of the invention;

FIG. 3 illustrates schematically the manner in which a recording sheetin accordance with the invention can be inscribed employing a printerunit including a scanning electrode and a source of ultraviolet light;

FIG. 4 is a view of a portion of the printer unit of FIG. 3 showing inparticular a multi-element electrode in contact with the recordingsheet;

FIG. 5 shows a multi-layer sandwich for electro photography applicationsin accordance with another embodiment of the invention, and parts of thelayers are broken away to better illustrate the construction;

sneaeea FIG. 6 is a schematic view showing the sandwich of FIG. in acamera and connected to a voltage source;

FIG. 7 is a cross-sectional view of the multi-layer sandwich of FIG. 5;

FIG. 8 is a cross-sectional view of a sandwich similar to that of FIGS.5 and 7 and having in addition a photoconductive layer; and

FIG. 9 is a cross-sectional View of a modified construction for thesandwich of FIGS. 5 and 7.

The basic chemical components of the recording system of the inventioninclude catalyst material, and a sensitive coloring material whichundergoes a photochemical change of color when contacted by the catalystand exposed to light. The sensitive coloring material is contained in asheet for facsimile and printing applications, and is contained in alayer of a multi-layer sandwich for photographic applications. Thecatalyst comes into contact with the sensitive material in an activeform only at the time when the mark or image is produced. Until thattime, the catalyst must not act on the sensitive material. This isaccomplished by separating the catalyst from the recording medium andapplying the catalyst to the sensitive material at the proper time. Thecatalyst is a metal, and the metal is converted to photosensitive metalions which are moved by an electric field to the recording medium. Thecatalyst does not affect the color of the sensitive material until it isreduced to the active free metal form. This reduction or activation ofthe catalyst is accomplished by exposing the photosensitive ions whichhave been deposited on the recording medium to light. Since the changeof the sensitive material to a contrasting color is also a photochemicalchange, the activation of the catalyst and the color change of thesensitive material can be accomplished simultaneously by a singleexposure to light.

In a particular message printing application, a scanning electrode ismade of the catalyst metal, and as the electrode moves across therecording sheet it is electrically pulsed in such a way thatphotosensitive metal ions are deposited on the recording sheet atselected points. By building up a pattern of these discrete deposits ofphotosensitive metal ions, characters made up of patterns of dots areformed on the sheet when it is exposed to light.

In photographic applications, photosensitive catalyst metal ions aremoved by an electric field from one layer of a sandwich structure to thesensitive coloring material which is in a different layer of thesandwich. The current which deposits the metal ions may be areamodulated so that in places where a comparatively large amount of metalis deposited the image will be dark and in places where less metal, ornone at all, is deposited, the image will be light.

Alternatively, the current may be unmodulated so that the photosensitivemetal ions are deposited uniformly, and the conversion of the metal ionsto the free metal form may be modulated by exposure to the light fromthe subject that is being photographed. In this case, the current iscontinued after the exposure to remove the residual metal ions from theimage forming layer, and then the image is developed by exposure tostrong unmodulated light which is rich in ultraviolet.

One of the most important aspects of this catalytic system is that eachatom of the catalyst metal which is brought into efiective contact withthe sensitive material as described above will act on a larger amount ofthe sensitive material. The sensitive material can be photochemicallychanged to a different valence state having a contrasting color. Thisphotochemical reaction is an extremely slow reaction in the absence ofcatalyst material, so the color change of the sheet occurs only in thoseplaces where the catalyst metal has been deposited and activated.

It should be noted that the reaction of the sensitive material to form acolored product is not electrolytic and therefore is not subject toFaradays law. Electrolytic current is used to deposit the metal ions onthe sensitive material, but once the metal ions are deposited andreduced to free metal, each metal atom catalyzes the photochemicalreaction of a larger amount of the sensitive material. Therefore, for agiven amount of current, a mark of greater color density can be achievedthan is possible with a wholly electrolytic system. Thus, the catalyticsystem of the invention provides amplification, or a gain insensitivity.

In order to incorporate the sensitive material in a conductive sheet orlayer, certain other components besides the catalyst and the sensitivematerial are employed. In one embodiment, the sensitive material isdispersed in a plastic binder, and the plastic is selected so that itcan be cast either as a separate film or as a coating on a conductivesupport. The plastic is impregnated with a highly ionizable saltdissolved in a slow-drying solvent which couples the salt to the plasticbinder. The salt erves as an electrolyte which renders the sheet highlyconductive. It has also been found desirable to include a stabilizingagent in the recording medium which serves to make the marks or imagemore permanent.

The sensitive material employed in the recording medium is preferably asolid, crystalline pigment, which is water insoluble, and which ischanged by a photochemical reaction to a lower or higher valence statewhich has a contrasting color. The preferred sensitive material istitanium dioxide (TiO which is an excellent white pigment. This pigmentwill darken very gradually upon exposure to ultraviolet light for a longtime, but the darkening is so slow as to be negligible when the exposureis for a limited time. When incorporated in a recording sheet, thetitanium dioxide pigment gives the sheet a white color which provides anexcellent background for inscription, and the white sheet will notdarken appreciably in daylight. Also, in the absence of a catalyst, thesheet is comparatively insensitive to other radiation such asbombardment by electrons or gamma radiation. It has been found that whentitanium dioxide crystals are contacted with traces of free metal in thepresence of light, the crystals darken very rapidly. Although themechanism of the color change is not fully understood as yet, it isthought that the dark color is produced by photoreduction of TiO at thesurface of the crystal to Ti O thus producing F centers in the crystalwhich give it a dark color. It is believed that the metal is adsorbed onthe crystal and catalyzes the photoreduction so that it takes place veryquickly.

The selection of the catalyst metal to be employed for facsimile andphotography applications is limited by the fact that the metal has to beapplied to the recording medium in a controlled manner as describedabove. For facsimile and printing applications, it is possible to makethe recording electrode of the catalyst metal and deposit ions of themetal on the sheet as previously mentioned. For photography, thecatalyst may be provided in the form of a layer of metal separated fromthe sensitive layer by a barrier layer, and metal ions are moved fromthe metal layer through the barrier layer to the sensitive layer byapplying voltage across the layers. However, the catalyst ions depositedon the recording medium must be reduced to the free metal or Zerovalance state in order to be effective. For this reason, silver, gold orlead is employed as the catalyst metal because ions of these metalsdeposited on the recording medium are reduced to free metal by light.Thus, there are really two photochemical reactions which take place inmarking the recording medium. First, the metal ions which are depositedon the recording medium are photochemically reduced to free metal, andthis free metal then catalyzes the photochemical color change of thesensitive material.

Metal oxides other than titanium dioxide have been found to change colorrapidly in the presence of ultraviolet light and metal such as silver.Examples are zinc oxide, aluminum oxide, lead oxide, tin oxide, vanadiumoxide, and molybdenum oxide. It has also been found that cuprousthiocyanate (CuSCN) is darkened when contacted by silver and exposed tostrong light in accordance with the invention. A class of materialswhich may be used as the sensitive material for the recording sheet ofthe invention is compounds of metals of groups 2, 3, 4, and 6 of thePeriodic Table after Mendeleev, which have a stable state ofcharacteristic color and which change to a lower or higher valence stateof contrasting color upon exposure to strong light for a long time. Twoor more compounds of the above class may be used in a mixture ifdesired.

The best results have been obtained using titanium dioxide as thesensitive material in the recording shee and silver as the metalcatalyst. Also, it has been found that it is preferable to expose therecording medium to light which is rich in ultraviolet for purposes ofdeveloping the marks or image on the recording medium. An ultravioletlamp is suitable for this purpose, but even bright daylight which isrich in ultraviolet will develop the marks or images satisfactorily.

For the hinder or carrier material, it is desirable to use a substancewhich can be rendered conductive. Suitable materials are high molecularweight organic substances (mol. wt. in the range of about 10,000 to100,000) which are water soluble and contain a large percentage of polargroups. Natural products within this classification are starches,dextrins, algins, gelatins and cellulose derivatives such as methylcellulose, hydroxyethylcellulose and carboxymethylcellulose. Syntheticsubstances of this class are polyfunctional plastics such as polyvinylalcohol, polyacrylic acid and polyvinyl pyrrolidones. The preferredbinder material is polyvinyl alcohol since this material is onlyslightly hygroscopic, and forms tough, smooth films which provide a goodsurface for recording.

The preferred electrolyte materials are ammonium salts and substitutedammonium salts. Examples of suitable ammonium salts are ammoniumnitrate, ammonium citrate, ammonium levulinate, ammonium silicofluorideand ammonium bifluoride. Examples of suitable substituted ammonium saltsare triethylenetetramine acetate or nitrate, tniethanolamine acetate ornitrate, piperidine nitrate, and dodecylamine acetate. Other electrolytesalts can he employed, but the salts named above have been found to becompatible with the other ingredients and they make the recording mediumhighly conductive. In selecting the electrolyte salt, attention must begiven to the chemical properties of the catalyst metal. For example,where the catalyst metal is silver, silver ions will be transported tothe recording sheet, and the electrolyte salt must not interfere withthe silver ions. For example, ammonium chloride is not used because thechloride ion will precipitate the silver ions before they have a chanceto act on the sensitive material. Of the electrolyte salts named above,8.1114 m-onium nitrate has given the most satisfactory results becauseof its high solubility and ionization, and also because of itscompatibility with polyvinyl alcohol.

The solvent couples the electrolyte salt to the plastic. The solventshould be very slow drying, it must be compatible with the bindermaterial that is employed, and it must be reasonably insensitive tovariations in humidity and temperature. The selected solvents areslightly hygroscopic and because of this they serve to plasticize thebinder material. In cases where the electrolyte salt itself issufficiently hygroscopic to plasticize the binder (such astriethanolamine acetate), it may be possible to omit the solvent.Suitable solvents are polyhydroxy alcohols having a molecular weightbelow about 200, such as glycerine, ethylene glycol, diethylene glycol,triethylene glycol and propylene glycol. Mannitol and so-rbitol are alsopossible solvents, but these are not as satisfactory as the others. Thepreferred solvent for use with the polyvinyl alcohol binder isglycerine.

It has been found that by including a stabilizing agent in the recordingmedium, the color density and contrast of the marks or image upon ageingis significantly improved. This stabilizing agent is an organiccomplexing compound which forms stable complexes with the changed formof the sensitive material. The preferred stabilizing agent is ammoniumsuccinate. This material improves the permanence of the mark or imageformed on the recording medium, improves resolution, and decreasesbleeding of the mark or image upon ageing. Examples of other suitablestabilizing agents are ammonium oxalate, cupferron, an ion exchangeresin known in the trade as AM9 (American Cyanamid), O-hydroxyquinolineand the ammonium salt of ethylenediaminetetracetic acid. A distinctivecharacteristic of these stabilizing materials is that they are allcomplexing agents for transition metal ions.

For facsimile applications, the recording sheet can be made in the formof a thin homogeneous film 10 as illustrated in FIG. 1. This film doesnot necessarily contain any paper or fibrous material. However, fibrousmaterial can be included in the composition, and this tends to give therecording sheet additional body and more of the appearance of paper.Still another way to make the recording sheet is to apply a coating 11of the recording medium on a conductive support 12. The support 12 canbe paper which has been impregnated with graphite or metal particles torender it conductive, or it can be a laminate of paper and metal foilsuch as is sometimes used for packaging purposes.

A typical composition and method for making recording sheets in the formof a film as in FIG. 1 is as follows:

The above ingredients are combined and agitated in a blender in thefollowing manner. The water, glycerine, ammonium nitrate and ammoniumsuccinate are mixed and agitated until the salts are dissolved. Then thepolyvinyl alcohol and antifoaming agent are added and agitation iscontinued until all of the constituents are blended. The titaniumdioxide is then blended in slowly with agitation until a smoothhomogeneous mix is obtained. The composition is milled in a ball millfor about one hour in order to reduce foam and further disperse theinsoluble titanium dioxide particles. The mixture is then poured onto asmooth surface such as a glass plate and is doctored with a blade or rodhaving about a .020 inch clearance between the glass surface and theedge of the doctoring tool. This will produce a film which dries to athickness of about .003 to .005 inch. Drying is best ac complished atroom temperature for about 24 hours. If oven drying is employed, thetemperature should not go above about F. because above this temperaturethe film may be distorted. When the film is dry, it can be peeled offthe glass, and it is tough and pliable. It has a white color, and isreadily markable with a silver electrode employing pulses having anamplitude of about volts, a pulse width of about 50 to 500 microsecondsand a pulse interval of about 1600 microseconds.

Variations in the proportions of the ingredients given above arepermissible as shown by the ranges stated in Table I set forth above.The ranges in the table are based on 300 milliliters (304 grams) of the10% aqueous solution of polyvinyl alcohol. For the glycerine, the rangeis primarily determined by the effect of humidity. Below the minimumamount of 6 grams, the film becomes too dry if the relative humidity isbelow about 30 percent. Above the maximum amount of about 25 grams thefilm becomes too wet if the relative humidity exceeds about 30 percent.

For the ammonium nitrate, using less than the minimum amount of gramsthe film is too nonconductive, and above the maximum amount of about 15grams the ammonium nitrate tends to crystallize on the film surface. Thevariation in the proportion of water employed is not critical since itschief influence is to control the viscosity of the composition so thatit can be cast as a film conveniently. The stabilizing agent has littleefiect below the minimum amount of 2 grams, and it tends to crystallizeon the film surface if used in amounts above the maximum of about 6grams. The sensitivity of the film decreases unduly if less titaniumdioxide is used than the stated mini mum of 45 grams, and above themaximum amount of 65 grams of titanium dioxide the film becomes slightlyabrasive such that marks may be formed on the film mere- 1y bycontacting it with the silver electrode Without applying voltage to it.Of course, this is not satisfactory. Also, when the film is too abrasivethe contact resistance of the electrode traveling across the film is toohigh and this reduces the efficiency of the system. When the preferredproportions of titanium dioxide and binder are employed, the titaniumdioxide particles are protected by the binder and the recording surfaceis smooth. The antifoarning material is used to reduce the tendency ofthe composition to foam as a result of the blending, and the amount usedis not critical.

The preferred proportions and ranges of the ingredients in the film ofFIG. 1 when it is ready for use are given in the following table on thebasis of percentage of the total composition by weight.

Table II Ingredients Preferred Range, Percentage percent SensitivePigment 50 40-60 Binder 25 20-30 Solvent 11 -15 Electrolyte Salt. 3 5-10Stabilizing Agenh-.- 4 2-5 Antifoaming Agent 2 As He quired.

As previously mentioned, it is possible to include fibrous material inthe composition. For this purpose, a finely divided wood cellulose fiberproduct such as that sold under the trademark Solka Floc can be used.The purpose of including this material in the film is to give the filmmore dimensional rigidity, prevent the film from sweating in conditionsof high relative humidity, and give the film the feel and texture ofpaper. The following composition which includes such fibrous material isgiven by way of example:

The recording sheet can also be made in the form of a laminate asillustrated in FIG. 2. The sheet 10 includes a plastic film 11 on aconductive support 12 which may be paper impregnated with graphite ormetal, and alternatively may be a paper-metal laminate. One satisfactoryway to impregnate paper with graphite is to spray it with or dip it in asolution of colloidal graphite. Such a solution is commerciallyavailable under the trademark Aquadag. The proportions of theingredients for the film when it is applied as a coating on a conductivepaper backing are somewhat different than given above, particularly withrespect to the glycerine content, because the paper absorbs a ratherlarge percentage of the glycerine and more must be used. The followingcomposition is given by way of example:

The ingredients are mixed together in a blender and then milled forabout one-half hour by the same procedure as described above inconnection with the description of the film of PEG. 1. The compositionis then ap lied to the graphite impregnated paper.

in providing the embodiment in which the plastic film is coated on ametal foil-paper laminate, the film has the same composition as when itis made without any backing. It has been found that the best adhesion isobtained when a very thin coating of polyvinyl alcohol is baked onto themetal foil, and then the sensitive composition is applied to the bakedcoating. The sensitive composition may be cast or sprayed onto the bakedcoating.

FIGS. 3 and 4 illustrate a portion of a printer unit with which therecording sheets of FIGS. 1 and 2 may be employed. The utility of therecording sheet is not restricted to any specific apparatus, and theprinter mechanism of FIGS. 3 and 4- is shown only for the purpose ofgiving an example of suitable apparatus. As illustrated in FIG. 3, therecording sheet 10 is a long strip which can be supplied as a roll. Thesheet 10 is moved past a scanning mechanism 26 by rollers 27 and 28which are driven by a motor through suitable gears (not shown). Themotor also drives one of the pulleys 29 and 30 of the scanning mechanism26. A continuous belt 31 is driven by the pulleys 29 and 3t and anelectrode unit 32 is carried by the belt 31 and moves across the widthof the sheet 11 once for each revolution of the belt. Additionalelectrode units may be spaced along the belt so that as one unit leavesthe right side of the sheet, another is in position at the left sideready to start the next line.

The electrode unit 32 is provided with several printing fingers 33 asillustrated in FIG. 4. These fingers are pieces of wire made of thecatalyst metal such as silver as described above. As viewed in FIG. 4,the right ends of the fingers 33 contact the recording sheet 10, and theleft ends of the fingers contact the conductors 34 which extend alongthe face of an insulating panel 36. The printing pulses are suppliedthrough the conductors 34 to the fingers 33, and the fingers apply thepulses to the recording surface of the sheet 19. Electrical contact maybe made to the back of the sheet it by a metal plate 37 which extendsacross the width of the sheet. Electrical contact may also be made tothe same side of the sheet as is in contact with the fingers 33 bymaking the roller 28 of conductive material.

A positive pulse applied to any one or" the metal fingers 33 causesmetal ions to be deposited on a very small area of the sheet It). Byapplying the pulses to the printing fingers according to a code, thesemetal deposits are formed in a pattern which corresponds to thecharacters of a message as illustrated in FIG. 1. The characters are:made visible by exposing the sheet to ultraviolet light, and for thispurpose a light source 38 may be provided in the printer unit asillustrated in FIG. 3. However, the record may be developed by exposingthe sheet to ul- 9 traviolet light after removing it from the printerunit, if desired.

The recording system of the invention can also be applied tophotography. Images have ben recorded employing a multi-layer sandwichstructure 13 as illustrated in FIGS. and 7. In this structure, there isa transparent laminated electrode 14 consisting of a transparent glassor plastic body 15 with a very thin coating 16 of material such as tinoxide which is conductive and also transparent. One electricalconnection 21 is made to the conductive coating 16. Transparentlaminates of this type are available commercially under the trademarksNESA and EC. A layer 17 containing the catalyst metal is in contact withthe electrode 14. Contacting the layer 17 there is a transparent barrierlayer 13, and an image forming layer 19 which contains a dispersion ofthe sensitive material contacts the barrier layer. Electrical contact ismade to the image forming layer 19 by the electrode 20. Electricalconnections 21 and 22 are made to the electrode layers 16 and 2t), andvoltage is applied across the electrodes from the voltage source 23.Current flows transversely through the structure and forms ions of thecatalyst metal in layer 17. Electrode layer 16 is made positive andlayer 19 is made negative so that these metal ions move through thebarrier layer 18 to the image forming layer 19 under the influence ofthe applied electrical field.

In order to record an image in the layer 19, the deposition of catalystmetal in the image forming layer can be modulated optically aspreviously mentioned. This process will be described with reference toFIGS. 5 to 7. The sandwich 13 is placed in a camera enclosure 40provided with a lens 41 and a shutter 42 as illustrated schematically inFIG. 6. The transparent electrode 14 faces the shutter 42 so that whenthe shutter is opened, light from the subject to be photographed will bedirected onto the sandwich 13 as represented by the arrows 43 and 44 inFIG. 6. With the shutter closed, direct current voltage is applied tothe sandwich from the source 23 by closing the switch 24 for a timesufficient to ionize and move all or nearly all of the metal catalystfrom layer 17 to layer 19. The barrier layer 18 is extremely thin and ispervious to the metal ions so that they quickly pass through the barrierand into the image forming layer 19. The metal ions have less mobilityin the image forming layer so that nearly all of the ionic catalyst isconcentrated near the upper surface of layer 19 adjacent the barrier ashort time after the voltage is applied to the sandwich.

Then the shutter is actuated to admit enough light from the subject toform a latent image. There is no longer any reflective metal layer inthe path of the light because the metal of layer 17 has migrated tolayer 19 and is now in ionic form. The light passes through thetransparent layers 15, 16 and 18 and is directed onto the image forminglayer 19. The light reduces the photosensitive metal catalyst ions tofree metal according to the intensity distribution of the light over thearea of the image forming layer 19. Thus, the density of the freecatalyst metal in layer 19 after the exposure varies over the area ofthe layer providing a latent image.

The current supplied from the source 23 may either be continued orstopped during the exposure. After the exposure, current is passedthrough the sandwich for a time sufficient to remove the residual metalions from the layer 19. These residual metal ions move through the layer19 and are deposited on the negative electrode 20. The catalyst whichwas reduced to free metal by the exposure is absorbed on the pigmentcrystals in the image forming layer and does not re-ionize to anyundesirable extent. When the residual metal ions have been removed fromlayer 19, the current flow is stopped. The image is developed byexposing the sandwich to strong light which is rich in ultraviolet, andthis causes the sensitive pigment to change to a contrasting color inthe places where it is in contact with the metal catalyst. In elementalareas of the layer 19 where the density of the catalyst metal ishighest, the contrast is greatest, and in elemental areas where there isno catalyst metal, the layer 19 retains its original color. When thesensitive coloring ingredient is titanium dioxide and the catalyst issilver, the original color of layer 19 is white and the contrastingcolor is black or dark brown. In between these two extremes there arevariations in the gray scale in accordance with the area distribution ofthe catalyst metal deposited in layer 19. The structure comprisinglayers 18-20 can be peeled off of the electrode 14 either before orafter developing the image, if desired.

A satisfactory construction for the sandwich 13 of FIGS. 5 and 7 will bediscussed by way of example. The transparent electrode 14 consisting oflayers 15 and 16 is available commercially as mentioned above. A verythin film of silver is vapor deposited over the conductive layer 16 ofthe transparent electrode to form the catalyst layer 17. Gold and leadmay also be used for the catalyst layer 17 as described above. Asatisfactory thickness for the layer 17 is about 50 angstrom units. Athin coating of transparent organic film-forming material such aspolyvinyl alcohol is applied over the catalyst layer 17 to form thebarrier layer 13. A satisfactory thickness for layer 18 is 1 micron. Thepolyvinyl alcohol adheres uniformly and intimately to the layer 17 andthus provides good electrical contact between the layers. The layer 18is dried, and then the recording medium is cast over layer 18 to formthe image forming layer 19. The composition of the recording medium isin accordance with the previous description, and the preferredcomposition is set forth in Tables I and II above. A suitable thicknessfor the layer 19 is about 1 mil. The electrode 29 serves the samefunction as the conductive backing 12 of the recording sheet of FIG. 2,and it may be made in any of the ways discussed in connection with FIG.2. The sandwich will operate satisfactorily if a coating of graphite ispainted over the layer 19 to form the electrode layer 20. If theelectrode 20 has a metal layer, it can be placed on the image forminglayer before it dries to insure uniform and intimate adhesion betweenthese layers. The conductors 21 and 22 are connected to the conductivelayers 16 and 20. It should be noted that the thickness of the layers isexaggerated in the drawings for purposes of illustration, and

the layers are not drawn to scale.

It has been found that it is also possible to record images with thecatalytic recording system of the invention by area modulating thecurrent flow through the sandwich. A sandwich 13a adapted for suchoperation is shown in FIG. 8. A layer 25 of photoconductive material isprovided between layers 16 and 17 of the sandwich 13a as shown in FIG.8. In all other respects, the sandwich 13a is the same as sandwich 13.With this arrangement, no mechanical shutter is needed because the timeof the exposure is controlled electrically. A voltage pulse is appliedacross the electrodes 16 and 20, and light from the subjectarea-modulates the conductivity of the photoconductive layer. Thecurrent which fiows transversely through the structure is area-modulatedin accordance with the intensity distribution of the light. Thus, theamount of catalyst metal which ionizes and moves to the image forminglayer 19 varies over the area of the sandwich to provide a latent image.This image is developed by exposing the layer 19 to strong light aspreviously explained.

The sandwich 13b of FIG. 9 is a modified construction in which noseparate photoconductive layer is provided, and in which the catalyst isincorporated in a carrier medium of plastic material. A satisfactorymethod for making the sandwich 13b illustrated in FIG. 9 is as follows.Silver nitrate in the amount of 10 milligrams is dissolved inmilliliters of a 7 percent aqueous solution of polyvinyl alcohol. Atransparent film of this material is cast on the NE-SA or EC glasselectrode 14 and is dried to provide the catalyst layer 17. A thintransparent film of polyvinyl alcohol is then applied over the catalystlayer 17 and is dried to form the barrier layer 18. Thirty grams oftitanium dioxide is dispersed in toluene and this mixture is ball milledfor about 4 hours. Forty-five grams of a plastic material known in thetrade as Pliolite is then added to the titanium dioxide and toluenemixture, and this substance is milled further until the titanium dioxidepigment is properly dispersed. The dispersion is then diluted withtoluene until the consistency is proper for casting purposes, and theresulting material is cast as a layer 19 over the polyvinyl alcohollayer 18. After the layer 19 has dried, a graphite electrode 26 ispainted on over the layer 19. The plastic layers 17, 18 and 19 provideuniform and intimate surface contact with the electrode layers 16 and 20so that there is good electrical contact between all layers of thesandwich.

Images can be recorded in layer 19 of the sandwich 13b of FIG. 9 in themanner described in connection with FIG. 6. That is, sflver ions areuniformly moved from layer 17 through layer 18 to the image forminglayer 19 by applying voltage across the electrodes 16 and 20 with thesandwich in the dark. Then the exposure is made to reduce some of thesilver ions to free silver, thus forming a latent image in the layer 19.The residual silver ions are removed from layer 19 by continuingthecurrent after the exposure, and the image is developed by directingstrong ultraviolet light onto the layer 19.

Tests conducted with the sandwich of the invention show that thetitanium dioxide pigment in the layer 19 exhibits photoconductivity.That is, the transverse conductivity of the layer varies with theintensity of light directed on it. The photoconductivity of the layer 19has little effect when images are formed by optical modulation asdiscussed above. However, images may be recorded with the sandwich ofFIG. 9 by using the photoconductivity of the layer 19 to area-modulatethe flow of catalyst ions from the layer 17 through the barrier layer 18to the image forming layer 19. In this case, the image is recorded inthe manner described in connection with FIG. 7 except that the titaniumdioxide in the layer 19 serves to area-modulate the current and noseparate photo-conductive layer is provided.

It is apparent that the catalytic recording system of the invention hasvarious applications, and in each of the applications it has certainadvantages over known systems. In facsimile and message printingapplications, one of the most important advantages of the catalyticsystem is that it is more sensitive than known systems. Because of thisit is possible to make the recording sheet substantially dry and yetmark it without sparking at high printing speeds and relatively lowvoltages. For example, messages can be printed in the apparatus of FIGS.3 and 4 without sparking at a speed of 20 inches per second and markingvoltages of 25 to 200 volts. The printing has satisfactory contrast,resolution and permanence. No wet processing is required either beforeor after marking, and to protect the sheet during storage, simplepackaging such as a plastic wrapper can be used. In photopraphyapplications, one of the main advantages of the catalytic system of theinvention over photographic film of the silver halide type is that therecording medium is not sensitive to light or other radiation eitherbefore or after the image is formed. This is particularly advantageousin conditions where the ambient radiation level is high. Anotheradvantage is that no wet chemical processing is needed to develop theimage, as is the case in silver halide photography. The catalyticrecording medium of the invention is more sensitive than entirelyelectrolytic recording mediums previously proposed forelectrophotography, and it can be used in a much drier condition thansuch electrolytic systems.

I claim:

1. A method of making visible records, including the steps ofelectrolytically applying ions of a metal selected from the groupconsisting of silver, gold and lead to portions of a recording sheetcomprising a compound selected from the group consisting of cuprousthiocyanate, and oxides of titanium, zinc, aluminum, lead, tin, vanadiumand molybdenum dispersed in an organic binder containing a polyhydroxyalcohol and a non-photosensitive nitrate salt which renders saidrecording sheet electrically conductive, said ions being applied to saidsheet by applying direct current electric potential from an externalpotential source to produce a potential difference between a conductiveelectrode which is in contact with said sheet and a solid elementcontaining the selected metal which is distinct from but in contact withsaid sheet to produce current through said sheet which transfers ions ofthe metal from said solid element to said sheet by electrolysis, andexposing said sheet and the metal ions thereon to light rich inultraviolet.

2. A method of making visible records, including the steps ofelectrolytically applying silver ions to a recording sheet whichcomprises titanium dioxide material dispersed in an organic bindercontaining a polyhydroxy alcohol and a non-photosensitive nitrate saltwhich renders said recording sheet electrically conductive, said silverions being applied to said sheet by applying direct current electricpotential from an external potential source to produce a potentialdifierence between a conductive electrode which is in contact with saidsheet and a solid element of silver material which is distinct from butin contact with said sheet to produce current through said sheet whichtransfers silver ions from said solid element to said sheet byelectrolysis, and exposing said recording sheet and the silver ionsthereon to light rich in ultraviolet.

3. A method of making visible records, which comprises moving arecording stylus comprised of silver material over and in contact with arecording sheet comprising titanium dioxide pigment dispersed in awatersoluble organic binder containing a polyhydroxy alcohol and anon-photosensitive nitrate salt which renders said recording sheetelectrically conductive, applying direct current electric potential froman external potential source to produce a potential difference betweensaid recording stylus while said stylus is in contact with said sheetand a conductive electrode while such electrode is in contact with saidsheet to produce current through said sheet which transfers silver ionsfrom said recording stylus to said sheet by electrolysis, and exposingsaid sheet and said silver ions thereon to light rich in ultraviolet.

4. A method of making visible records, which comprises providing arecording medium in the form of a plastic sheet comprising a binder ofwater-soluble plastic of the polyvinyl type, a polyhydroxy alcohol, anonphotosensitive nitrate salt, and a compound selected from the groupconsisting of cuprous thiocyanate and oxides of titanium, zinc,aluminum, lead, tin, vanadium and molybdenum, electrolytically applyingto said sheet ions of a metal selected from the group consisting ofsilver, gold and lead by applying direct current electric potential froman external potential source to produce a potential difference between aconductive electrode in contact with one side of said sheet and a solidelement containing the selected metal in contact with the other side ofsaid sheet to produce current through said sheet which transfers ions ofthe selected metal from said solid element to said sheet byelectrolysis, and exposing said sheet and said metal ions thereon tolight rich in ultraviolet.

5. A method of making visible records which comprises, providing arecording medium in the form of a layer of an organic binder containingtitanium dioxide and a nonphotosensitive nitrate salt and a polyhydroxyalcohol distributed through an area thereof, electrolytically applyingsilver ions to said recording medium from a solid layer containingsilver which is distinct from but in contact with said recording mediumby applying direct current electric potential from an external potentialsource to produce a potential difference between said silver-containinglayer and an electrode in contact with said recording medium and therebytransfer silver ions from said solid layer to said recording mediumthroughout said area thereof, exposing said recording medium to apattern of light from a subject for forming a latent image in saidrecording medium by reducing silver ions in said area to silver metal inaccordance with the intensity distribution of the light, subsequentlyremoving residual silver ions from the recording medium by passingelectric current between electrodes on opposite sides of said recordingmedium, and exposing said recording medium to light rich in ultravioletwhich has a substantially uniform intensity distribution over the areaof said recording medium, with such light being effective to produce avisible color change in said recording medium corresponding to saidlatent image.

6. A recording sheet having a recording medium in the form of a layercomprising a polyvinyl plastic binder material, a polyhydroxy alcoholmaterial having a molecular weight less than about two-hundred and anon-photosensitive nitrate salt distributed in said binder material forrendering the same electrically conductive, and a compound selected fromthe group consisting of cuprous thiocyanate and oxides of titanium,zinc, aluminum, lead, tin, vanadium and molybdenum distributed in saidbinder in an amount sufiicient to produce a color change in elementalareas of said recording medium when recording signals are appliedthereto with a stylus electrode made of a metal selected from the groupconsisting of silver, lead and gold and the recording sheet is exposedto light rich in ultraviolet.

7. A recording sheet having a recording medium in the form of a layercomprising a binder of polyvinyl plastic, a polyhydroxy alcohol materialhaving a molecular weight less than about two-hundred and anon-photosensitive nitrate salt distributed in said plastic binder forrendering the same electrically conductive, and titanium dioxidedispersed in said binder in an amount sufficient to produce a colorchange in elemental areas of said recording medium when silver ions areapplied to said medium and said recording sheet is exposed to light richin ultraviolet.

8. A recording sheet having a recording medium in the form of a layer,comprising a binder of polyvinyl alcohol, a mixture of glycerine andammonium nitrate distributed in said binder for increasing theelectrical conductivity thereof, titanium dioxide material dispersed insaid binder in an amount sufficient to produce a color change in saidrecording sheet when silver is applied thereto and said sheet is exposedto light rich in ultraviolet, and said binder further containingammonium succinate therein in an amount sufiicient to stabilize such acolor change in said sheet.

References Cited in the file of this patent UNITED STATES PATENTS2,229,091 Kline Ian. 21, 1941 2,283,558 Kline May 19, 1942 2,294,146Wise Aug. 25, 1942 2,310,946 Finch Feb. 16, 1943 2,319,765 Talmey May18, 1943 2,339,267 Hogan et al. Jan. 18, 1944 2,476,800 Blackburn July19, 1949 2,528,005 Kline Oct. 31, 1950 2,776,251 Schwartz Jan. 1, 19572,901,348 Dessauer et al Aug. 25, 1959 2,983,654 Dalton May 9, 19613,052,541 Levinos Sept. 4, 1962 3,072,541 Shely et al. Jan. 8, 19633,088,883 Robillard May 7, 1963 FOREIGN PATENTS 188,030 Great BritainOct. 23, 1922 464,112 Great Britain Apr. 12, 1937 537,593 Great BritainJune 27, 1941 113,072 Australia May 22, 1941

1. A METHOD OF MAKING VISIBLE RECORDS, INCLUDING THE STEPS OFELECTROLYTICALLY APPLYING IONS OF A METAL SELECTED FROM THE GROUPCONSISTING OF SILVER, GOLD AND LEAD TO PORTIONS OF A RECORDING SHEETCOMPRISING A COMPOUND SELECTED FROM THE GROUP CONSISTING OF CUPROUSTHIOCYANATE, AND OXIDES OF TITANIUM, ZINC, ALUMINUM, LEAD, TIN, VANADIUMAND MOLYBDENUM DISPERSED IN AN ORGANIC BINDER CONTAINING A POLYHYDROXYALCOHOL AND A NON-PHOTOSENSITIVE NITRATE SALT WHICH RENDERS SAIDRECORDING SHEET ELECTRICALLY CONDUCTIVE, SAID IONS BEING APPLIED TO SAIDSHEET BY APPLYING DIRECT CURRENT ELECTRIC POTENTIAL FROM AN EXTERNALPOTENTIAL SOURCE TO PRODUCE A POTENTIAL DIFFERENCE BETWEEN